Bar screens have proven particularly valuable in sorting materials which have unequal dimensions. Wire or punched screens are typically used to sort materials of a granular nature in which all three dimensions are approximately equal. However, many classes of objects, including two of particular commercial interest, wood chips and municipal or industrial trash, are not readily amenable to separation by conventional screening processes.
In the manufacture of paper, logs are reduced to wood chips by chipping mechanisms, and the chips are cooked with chemicals at elevated pressures and temperatures to remove lignin. The chipping mechanisms produce chips which vary considerably in size and shape. For the cooking process, which is known as digesting, it is desirable that the chips supplied have a uniform thickness in order to achieve optimal yield and quality. Ideally, the supplied chips will allow production of a pulp which contains a low percentage of undigested and/or overtreated fibers. Thus, a means is needed to separate chips on the basis of thickness rather than any other dimension. Bar screens have proven particularly adept at separating materials based on a single dimension such as thickness.
With the rise in the recycling culture, a strong demand for an apparatus for separating municipal and industrial trash into its constituent components for recycling has developed. Conventional separation systems which utilize rotating screen drums have proved ineffective. Municipal trash which typically contains a certain portion of stranded material as well as sheet-like materials tend to clog the screens. Further, the tumbling action of screens can result in the breakage of components of the municipal waste stream such as glass bottles, thereby increasing the difficulty of recycling them. Bar screens consist of two sets of generally rectangular bars which are joined together in an array of racks. The two sets of bars are interleaved to form a screening bed. The bed consists of the elongated, rectangular bars and the narrow, rectangular spaces between the bars. Material to be sorted is introduced to the surface of the bed and the bars are caused to oscillate so that when one set of bars is going up, the other set is going down. This oscillatory motion tends to tip wood chips or other relatively small planar objects on edge so that those of a given thickness may slide through the gaps between the bars. Alternatively, it has been found when separating office waste paper, that bar screens prove effective in removing extraneous litter from the recovered office paper.
If the limitations of current bar screens could be overcome, the utility of the bar screen, already a valuable tool in the pulp industry and in the recycling industry, would be greatly increased.
One important improvement needed in bar screens is ways to attach thinner bar to the oscillation mechanism which causes the bars to oscillate.